1. The Field of the Invention
The present invention relates to vertical cavity surface emitting lasers. More particularly, the present invention relates to single mode vertical cavity surface emitting lasers that use photonic crystals with a central defect.
2. Background and Related Art
Vertical cavity surface emitting lasers (VCSELSs) are an example of semiconductor lasers used in fiber optical systems and have several advantages over other types of semiconductor lasers. VCSELs can be manufactured in large quantities due to their relatively small size and can often be tested on a single wafer. VCSELs typically have low threshold currents and can be modulated at high speeds. VCSELs also couple well with optical fibers.
VCSELs are typically made from both GaAs semiconductor materials and InP semiconductor materials, but GaAs semiconductor materials make better multi-layer mirror systems than InP semiconductor materials. A high reflectivity mirror system is needed in a VCSEL because the light resonates in a direction that is perpendicular to the pn-junction. The cavity or active region of a VCSEL is thus relatively short and a photon has little chance of stimulating the emission of an additional photon with a single pass through the active region. To increase the likelihood of stimulating the emission of photons, VCSELs require highly efficient mirror systems such that a photon can make multiple passes through the active region. The reflectivity is currently achieved using Distributed Bragg Reflector (DBR) layers.
One problem associated with VCSELs is related to the wavelength of the light that is generated. Current VCSELs typically generate light that has a wavelength of approximately 0.85 microns. This wavelength is primarily useful in very short distance fiber optic communications but is typically inadequate for longer distance fiber optic networks such as telecommunication networks. Attempts to develop and fabricate VCSELs that operate at longer wavelengths (1.3 microns and 1.55 microns, for example) and at higher power have proven to be very difficult. This difficulty is related to the fact that InP semiconductor materials permit the growth of a suitable active region for generating longer wavelengths, but the InP DBR layers are not effective. When GaAs semiconductor materials are used, the growth of the DBR layers is straightforward, but the active region is unsuitable. In addition, attempts to increase the power produced by VCSELs results in multimode emission. The difficulty in fabricating and designing the multiple DBR layers and the need for the lattice structures of the various layers in the VCSEL to match are additional reasons that impede the successful creation of a high power single mode VCSELs. Typically, attempts to create such VCSELs have resulted in VCSELs that produce insufficient power, are unreliable, or generate multiple modes.